This invention relates to circuits for the conversion of analog signals to digital signals, which circuits are powered with D.C. (direct current) voltages derived from power supplies energized with A.C. (alternating current) voltage such as 50 or 60 Hertz voltage and, more particularly, to the construction of such a circuit with dual slope integration over a time interval commensurate to the periods of the two A.C. voltages to reduce the effect of noise on output signals of the converter circuit from the A.C. voltages.
Analog-to-digital converters are employed frequently in the measurement of signals, such as those of biological experiments, by way of example, to convert analog values of measured signals to digitally formatted signals suitable for processing by a digital computer. Many forms of these circuits are known. It is common practice in the construction of these circuits to employ D.C. voltages to operate components of the circuits, such as transistors and integrated circuits from which these circuits are constructed. Ideally, the digital converter should be powered by a battery so as to avoid 50 or 60 Hertz hum, or similar noise which enters into power supplies which employ A.C. voltage as the primary source of power. However, in many situations it is necessary to employ A.C. power as the primary source of power to be converted to a regulated D.C. power which is then applied to operate the analog-to-digital converter. As a result, during measurements of high precision, such as may be employed in biological testing, the desired precision is reduced due to noise influence from the A.C. power.